374 I. Bowman — Physiography of the Central Andes. 



The dip and strike are, as a rule, so irregular in value that both 

 specific and general values are almost without interest. In 

 general it may be said that the prevailing dip of the sand- 

 stones along the lower Juntas valley north and northeast 

 of Cochabamba is northwest or toward the axis of the 

 eastern highland, though there are frequent and important 

 exceptions to this generalization. Of frequent occurrence are 

 zones of crushing where incredibly minute and numerous dis- 

 turbances have resulted from the adjustment of great block- 

 like masses of sandstone on either side. There is notably 

 greater textural firmness to the sandstones and shales toward 

 the west ; those toward the east are often so pliable as to fall 

 to pieces readily, though there are again many local excep- 

 tions to this condition. The whole sandstone series is unfossil- 

 iferous so far as we examined it and is marked throughout by 



Eio Chapare, eastern Bolivia, for fifty miles below Santa Rosa. 



the development of coarse conglomeratic deposits interstratified 

 with finer textured layers. In many localities cross-bedding 

 may be observed, and this feature, while not everywhere dis- 

 played, is about as common in one section as another. An 

 unusual feature of the easternmost sandstone on the margin of 

 the plains was the presence in the parting planes of small clay 

 lenses a foot or so in length and a few inches thick. The clay 

 is moderately dry and soft and falls out of a broken fragment 

 of sandstone as a separate unit. These clay lenses, from their 

 position and the frequency of their occurrence, appear to 

 indicate a general process of flood-plain erosion whereby a clay 

 layer resting upon sand was eroded to the point where only 



